1. Field of the Invention
The present invention provides a cycloidal mass spectrometer, which has inner and outer electrodes defining an annulus for passage of an electron beam therethrough and, more specifically, it provides such a cycloidal mass spectrometer, which permits a reduction in the number of electrodes and size, as compared with prior art cycloidal mass spectrometers.
2. Description of the Prior Art
The use of mass spectrometers in determining the identity and quantity of constituent materials in a gaseous, liquid or solid specimen has long been known. It has been known, in connection with such systems, to analyze the specimen under vacuum through conversion of the molecules into an ionic form, separating the ions by mass to charge ratio, and permitting the ions to bombard a detector. See, generally, U.S. Pat. Nos. 2,882,410; 3,070,951; 3,590,243; and 4,298,795. See, also, U.S. Pat. Nos. 4,882,485 and 4,952,802.
In general, mass spectrometers contain an ionizer inlet assembly wherein the specimen to be analyzed is received, a high vacuum chamber which cooperates with the ionizer inlet, an analyzer assembly which is disposed within the high vacuum chamber and is adapted to receive ions from the ionizer. Detector means are employed in making a determination as to the constituent components of the specimen employing mass to charge ratio as a distinguishing characteristic. By one of many known means, the molecules of the gaseous specimen contained in the ionizer are converted into ions, which are analyzed by such equipment.
It has been known with prior art cycloidal mass spectrometers to use a simple fixed collector and ramped electric field in looking at only one mass to charge ratio at a time. In many prior art mass spectrometer systems, regardless of whether they were of the cycloidal type or not, the ionizers were quite large and, as a result, dominated the design and specifications of the systems to be employed therewith.
U.S. Pat. No. 5,304,799 discloses a cycloidal mass spectrometer having a housing defining an ion trajectory volume, an electric field generator for establishing an electric field within the ion trajectory volume and an ionizer for receiving gaseous specimens to be analyzed and converting the same into ions, which travel through orthogonal electric and magnetic fields and subsequently impinge on a collector. This spectrometer was designed to have a plurality of different ions mass to charge ratios impinging on the collector generally simultaneously. It was stated that the cycloidal mass spectrometer and ionizer may be miniaturized to as provide a small readily portable instrument.
Cycloidal mass spectrometers belong to the so-called crossed field spectrometer group. In such spectrometers, charged particles move in magnetic and electric fields that are perpendicular to each other. In a uniform magnetic field as shown in FIG. 1, a charged particle moves in a circular path 2 determined by its mass, its charge, its speed and the magnetic field strength. The magnetic field may be established by pole pieces 3,4, the magnetic field as shown is parallel to the z axis and the electrical field is perpendicular thereto. The magnetic field may be generated by either a permanent magnet or electromagnet. The cycle""s frequency is determined by the time periods of the particle returning to a point in its trajectory. If a uniform electric field is imposed, normally across the magnetic field, the motion of the particle is imposed by a uniform motion rectangular to both fields as shown in FIG. 2. In this figure, the magnetic field is parallel to the z axis and the electric field is parallel to the y axis.
A particle of a given mass will cross a reference plane at equivalent locations that are separated by a fixed distance, which is designated the pitch of the periodic motion. Particles with different molecular weights return at different pitches to equivalent points in their trajectory, which is the separation effect of this type of mass spectrometry. An example of such separation and travel is shown in FIG. 3.
Cycloidal mass spectrometers of the prior art are generally based on the uniformity of the fields that result in a circular motion imposed by a linear motion of the charged particles.
The present invention focuses on field structures of a cycloidal mass spectrometer wherein the circular motion is imposed by another circular motion, thereby providing circular symmetry as shown in FIG. 4.
The present invention has provided a number of improvements in cycloidal mass spectrometers by providing a circular cycloidal mass spectrometer having a generally circular outer electrode and an inner electrode having a generally circular outer periphery. An ion-receiving annulus is defined in between the outer electrode and the inner electrode with the electrodes being structured to create an electric field therebetween. A magnetic field generator is structured to create a magnetic field oriented generally perpendicular to the electric field. An ion beam source for introducing ions into the annulus for travel therearound is provided. An ion exit for discharge of the ion from the annulus and an ion collector for receiving the discharge ions are provided.
In one embodiment, the inner electrode is generally cylindrical and of solid cross-section and in another it has a hollow interior. The ion beam source and ion exit are so positioned that, with respect to the annulus, that the ions travel circumferentially, preferably, at least about 45 degrees between entry and exit to obtain the desired multiple cycloid effect. The upper limit of travel can be any desired angle.
The structure and applied electric and magnetic fields may be such that the ions travel in a path that is like a higher order cycloid, such as an epicycloidal or hypocycloidal path.
The electric field may have a plurality of concentric equal potential circular field lines, each having a potential proportionate to the distance from the center of the mass spectrometer such that the field increases with increasing distance from the center.
It is an object of the present invention to provide a cycloidal mass spectrometer having a circular, elliptical or other suitable configuration and providing highly efficient operation.
It is a further object of the present invention to provide a circular cycloidal mass spectrometer wherein the number of electrodes employed to create the electric field may be reduced as compared with prior art linear configurations.
It is a further object of the present invention to provide such a circular cycloidal mass spectrometer which has reduced dimensions as compared with prior art mass spectrometers.
It is yet another object of the present invention to provide a circular cycloidal mass spectrometer which is adapted to be employed for Fourier transform mass spectrometry.
It is yet another object of the present invention to eliminate the need for stacked electrically conductive plates, through the use of circular configuration, in a cycloidal mass spectrometer.
It is yet another object of the invention to provide such a system wherein neither the starting energy nor the starting angle of the ions influences the character of the trajectory.